1. Field of the Invention
The present invention relates to a vehicle detecting system to detect the existence, passage, and the like of a vehicle. This system is used in a traffic control system to smooth the traffic flow by, for example, counting the number of vehicles which pass a predetermined place on the road and controlling the signals on the basis of the count number obtained.
2. Description of the Prior Art
As typical of conventional vehicle detecting systems, there has been known a system including an almost square loop coil buried under the road surface. A high frequency exciting current flows through the loop coil. When the vehicle passes above the loop coil, the inductance of the coil changes, so that the value of the current also varies. The passage of the vehicle can be recognized by detecting the change in this current.
However, such a conventional vehicle detecting system has the following problems.
In general, the loop coil has the size of about 2 m.times.2 m and the road must be dug up over a wide range to bury such a large loop coil into the road. Such a burying construction is large-scaled, so that the construction expenses increase and much labor is needed for the construction.
The loop coil buried under the road surface is frequently subjected to the loads in association with the passages of vehicles, so that the disconnection of the coil is likely to occur. The occurrence of the disconnection disenables the detection of vehicles.